As semiconductor device geometries approach 0.25 .mu.m feature size, increased attention has been directed to the difficulty in depositing aluminum or aluminum alloy into small vias holes or trenches. The use of multiple levels of interconnect metallization on a semiconductor device and shrinking feature size means that surface planarity, i.e., the flatness of the wafer surface, becomes increasingly important at critical operations in the processing of the device.
To improve surface planarity, various process steps and/or combinations of materials have been used. Years ago, it was standard practice on a multilevel metal structure to use sputtered aluminum or aluminum alloy in a trench or to form a via, i.e. an electrical connection between two levels of metal. This was done with the wafer surface between room temperature and 400.degree. C.
The step coverage of the conductor deposited in this manner was found to be poor, leading to reliability problems due to the thinning of the conductor on the sides and bottom of the trench, or the sides of the via hole. This problem became progressively worse as additional layers were put down over an already non-planar surface.
An approach to solving this problem has been to use a separate material, such as tungsten, to form a via or plug. The excess tungsten on the surface of the wafer and not in the via hole or trench is etched or polished away, leaving a planar surface on top of which the next level of metal is deposited.
Another problem facing the semiconductor industry as the dimensions of interconnect lines continue to shrink is the etching of the conductor to form the patterned lines. The standard method in the integrated circuit industry involves depositing a blanket metal film, lithographically patterning the film, reactively ion etching to form lines, and encapsulating the lines with an oxide. Pitted sidewalls and residual polymer present reliability and contamination problems after the metal etch. These could be alleviated by a damascene approach to forming lines. The damascene approach differs from standard methods since the oxide is deposited, lithographically patterned, and etched to form trenches. Then the metal interconnect is deposited into these trenches. Excess metal is polished using special chemical-mechanical polish methods to leave lines of interconnect encapsulated on three sides with oxide. Damascene processing eliminates the need for metal etch and is expected to become standard practice in future technologies.
A need exists to fill trenches and via holes with sputtered aluminum or aluminum alloy. This has been accomplished thus far by increasing the temperature of the wafer during deposition of the conductor to between 450 to 600.degree. C. At these temperatures (near the aluminum and aluminum alloy melting point), the conductor becomes softer, and can thus flow into small holes, completely filling them.
The mechanisms that lead to aluminum and aluminum alloy filling are not well understood. There are two general types of high-temperature processing: 1) reflow, and 2) hot Al. They are similar in that they both rely on temperatures between 400 to 600.degree. C. The so-called reflow process is typically done by depositing the aluminum or aluminum alloy at a temperature between room temperature and 400.degree. C. Afterward, the wafer is heated to between 500 to 600.degree. C. The wafer is typically held at this temperature for several minutes, allowing the conductor to flow into the trench or via hole. The hot Al process is carried out in several steps. In the first step, a portion of the desired thickness of aluminum or aluminum alloy is deposited at relatively cold temperature (less than 400.degree. C.), and then the wafer is heated up to between 500.degree. C. to 600.degree. C., and the remaining aluminum or aluminum alloy is deposited hot.
In either case, it is generally accepted that it is necessary to use a titanium wetting agent directly under the hot or reflowed conductor. It is very important that this titanium layer not be exposed to air before the conductor is deposited, as it will oxidize. In the absence of such titanium, the conductor will not flow or properly deposit in the bottom or on the sides of the trench or hole, leading to a void in the conductor.
At temperatures above 350.degree. C., titanium and aluminum react, consuming a portion of the aluminum line up to three times the thickness of the original titanium underlayer. For example, if 500.ANG. of titanium 20 is deposited under 5000.ANG. of aluminum 22 (FIG. 1) in a recess 24 in a dielectric 26, and is fully reacted to form TiAl.sub.3, a structure will result that is 2000.ANG. thick TiAl.sub.3 28 under 3500.ANG. aluminum 30. (See FIG. 2).
While it is generally accepted that the presence of titanium is needed for via hole or trench filling, the formation of TiAl.sub.3 28 causes some problems (see FIG. 3). First, the formation of TiAl.sub.3, which has a lower electrical conductivity than aluminum or aluminum alloy 30, reduces the amount of current-carrying cross-section that is composed of the low resistivity aluminum and increases the electrical resistance of the metal interconnect lines. Second, TiAl.sub.3 is difficult to chemically-mechanically polish: so it is desired to have this TiAl.sub.3 layer as thin as possible or eliminate it altogether. Third, the reaction to form TiAl.sub.3 during the hot deposition or reflow process may retard aluminum or alloy flow, and hence retard the filling process. This retardation arises from the TiAl.sub.3 "spiking" through the grain boundaries in the aluminum or alloy by preferentially growing into the aluminum grains at the grain boundaries. Thermodynamically speaking, the formation of TiAl.sub.3 at the grain boundaries requires less surface energy than growth of TiAl.sub.3 into the bulk of the aluminum or alloy grain. The kinetics of TiAl.sub.3 formation, however, provide a loophole or process window in which titanium can be used to provide the necessary wetting property. By depositing the aluminum or aluminum alloy cold initially, the rate of TiAl.sub.3 formation is reduced, and an adequate, smooth seed layer of conductor is provided for subsequent deposition. Control of the titanium under layer thickness becomes critical. Too little, and there will not be enough of a wetting layer to enable the hole or trench filling. Too much, and the reaction forming TiAl.sub.3 will impede the hole or trench filling. Normal variations in hole or trench size, depth and shape, titanium deposition rate and uniformity make precise control of the titanium thickness in and around the via hole or trench difficult, leading to yield and reliability problems.
Discontinuity of a sputtered, or PVD, titanium layer along a sidewall of a via or trench results in a poor physical barrier between the conductor and the interlayer dielectric oxide. Should the oxide outgas, the outgassing species may pass through the titanium layer when the wafer is heated for subsequent deposition steps. Hot aluminum or alloy is very sensitive to sputtering environment and surface conditions, and with any breach in the continuity of the titanium barrier/wetting layer, the outgassing species will negatively affect the fill of the aluminum or aluminum alloy.